This invention relates generally to hybrid cone beam image reconstruction, especially image reconstruction in volumetric computed tomography (VCT).
Diagnostic computed tomographic (CT) images are acquired in both axial and helical scans in clinical applications. In both single detector-row CT (SDCT) and multiple detector-row CT (MDCT), a helical scan can provide better longitudinal spatial resolution, faster patient throughput and better patient comfort relative to an axial scan because the patient table proceeds continuously and smoothly during the scan. In cone beam VCT, in addition to the benefits mentioned above, the helical scan can provide better image quality than an axial scan because it satisfies the so-called data sufficiency condition. Therefore, helical scans have played a dominant role in most clinical applications using SDCT, MDCT and VCT.
In a helical scan of SDCT, if projection data corresponding to a 360° view angle range are utilized to reconstruct one image, the z-location of an image plane is usually determined by an interception of the image plane and the helical source trajectory, which is located at the mid-way of the 360° view angle range. Thus, if a scan, as represented by the motion 102 of a radiation source 14 around a slice 104 of an object 22 in prior art FIG. 1, spans only one helical turn corresponding to 360° in view angle range (i.e., a single helical turn), only one image is reconstructed. Referring to prior art FIG. 2, if more than one image (of, e.g., a plurality of slices 104) is to be reconstructed, the helical scan 102 has to span more than one turn (i.e., multiple helical turns), in which each image plane or slice corresponds to a 360° view angle range. Consequently, the total projection view angle range is determined by the union of a family of 360° view angle ranges corresponding to each image plane. The 360° view angle ranges for each image plane overlap one another substantially, and the first image 106 and the last image 108 are prescribed at the locations that are indented by one half helical turn from the starting point 110 and ending point 112 of the scan, respectively. The indention in image location on each end of the scan corresponds to a 180° view angle range. Due to the indentions, the imaging zone (106-108) is smaller than the scan zone (110-112), and the total difference between these zones corresponds to a view angle range of 360°. The difference between the scan zone and the image zone is hereafter referred to as the over-beaming zone. If the helical source trajectory is mathematically expressed as
                                          ST            ⁡                          (              β              )                                =                      (                                          R                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                β                            ,                              R                ⁢                                                                  ⁢                cos                ⁢                                                                  ⁢                β                            ,                                                H                                      2                    ⁢                    π                                                  ⁢                β                                      )                          ,                                  ⁢                  β          ⊆                      [                                          β                s                            ,                              β                e                                      ]                          ,                            (        1        )            where β is an angle of rotation of radiation source 14, βs is the starting angle of the scan, βe is the ending angle of the scan, R is the radial distance of radiation source 14 from a central axis of the scan, and H is the distance proceeded by the patient table during one helical turn. The scan zone is [βs, βe]. The image zone can be defined as [βs+π, βe−π]. Note that the total dimension of the over-beaming zone along the z-direction is equal to H.
Prior art helical scans in both MDCT and VCT have so far used similar strategies for determining scan zone and image zone in diagnostic CT imaging. However, modern MDCT and VCT are being produced with increasing numbers of detector rows. As the number of detector rows increases, the over-beaming zone increases linearly and must be addressed appropriately. For example, in an SDCT embodiment having a detector row width of 0.625 mm, a typical helical scan may be carried out at pitch 1:1. The over-beaming zone is equal to the distance proceeded by the patient table during one helical turn, i.e., 0.625 mm in this example. A typical helical scan may be carried out at pitch 16/16:1 using a 16 detector-row MDCT with an identical detector row width, 0.625 mm. The over-beaming zone in this MDCT example is 16×0.625=10.0 mm. Thus, anatomic structures within a 10.0 mm zone may be irradiated by x-ray source 14 in a 16 detector-row MDCT, but no images corresponding to the structures in this zone are reconstructed. With even larger number of detector rows utilized in cone beam VCT, e.g., 64 detector row at width 0.625 mm, the over-beaming zone may increase to 64×0.625=40.0 mm if the helical scan is conducted at pitch 64/64:1.
Diagnostic volumetric CT will ultimately be provided by imaging system embodiments having even larger numbers of detector rows. Therefore, if no appropriate measures are exercised, the x-ray dose rendered to the over-beaming zone becomes significant from the perspective of ALARA (as low as reasonably achievable) principle.